Skip to main content

Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

See other formats


GAUANO AND BLACK GLACIERS                               91
such as existed on the Galiano in 1890, but some of the bushes are quite mature. A noteworthy difference between the two glaciers is the fact that the lower Black Glacier is a ridge, with a well-defined marginal valley on either side; and a still more notable difference is the absence of a pronounced piedmont bulb at the lower end of the Black Glacier. It barely passes beyond the mountain front and expands slightly at the very lowermost end; but no piedmont bulb is produced.
Although the earthquakes of September, 1899, must have shaken the Black Glacier as much as the Galiano, we have been unable to find any evidence of an advance; and there are several facts which prove that if this glacier did respond at all to the influence of earthquake shaking it was only to a very limited extent. Russell photographed the glacier from its alluvial fan in 1890, and in 1905 we re-occupied the site of this picture and were unable to detect any distinctive change in appearance. That the glacier has not advanced to any extent since 1890 is proved by the presence of a mature cottonwood forest about 250 yards from the glacier front, which also appears in Russell's picture. This photograph (PL XXXV, B) was taken at such a distance that we cannot be certain of the space that then separated the glacier from the forest, but it seems to have been about the same in 1890 as in 1905. Instead of advance, Black Glacier front gives evidence of recent recession, for in 1905 there was a low, moraine-covered stagnant ice mass about 100 feet from the glacier front, and between it and the glacier there was no mature alder growth. But just outside this stagnant ice mass, and within a few feet of it, were alder bushes from ten to fifteen years old. The interpretation that we place on these facts is that in 1890 the ice front stood where the stagnant ice now rests, and that since then there has been recession of about 100 feet. Prior to 1890 the ice had been receding over the area of about 200 yards now covered by alder, and for at least a half century had not been farther out than the border of the cottonwood forest.
Confirmatory evidence of the conclusion that Black Glacier has not been subjected to a recent advance as a result of earthquake shaking is supplied by two facts already mentioned, namely, the mature alder growth still standing on the glacier, and the ridge form of the glacier itself. The ridge extends about a mile from the ice front, to an elevation of about 1100 feet, and its surface rises up the valley at a high angle, averaging about 20 in the first half mile. It is a single, narrow, moraine-covered axial ridge, sloping toward the valley walls often at an angle of 30 or 35. Bare ice shows in many places, and stones frequently roll down the marginal slopes; but the moraine on the ridge top is thicker, 5 or 6 feet, and ice is rarely revealed there. Alders grow in this thick moraine, one that was cut down in 1905 being 18 years old. Some are killed by sliding and slumping of the soil, but this did not seem to be due to ice movement, for there were no crevasses in the central ridge in the lower half mile of the glacier. Farther up the glacier, where crevasses are present, deep morainic soil and alder bushes are not found. Alders 5 or 6 years old grow on the glacier up to 1000 feet, a mile from the end of the glacier. Beyond this the ridge flattens and disappears but in both 1905 and 1909 ablation moraine covered the glacier clear to the head.
Had there been notable recent advance of this glacier the ridge would of necessity have been destroyed and the glacier put into closer contact with the mountain wall. It is inconceivable that so pronounced a ridge could have been formed by the development of marginal valleys in the interval between 1899 and 1905. Also, the mature alders could not have survived an advance and breaking of any considerable amount. Those that are